Conventional gas turbine engines for powering aircraft include turbofan and turboprop engines. A recent improvement over these engines is the unducted fan engine such as disclosed in the above-referenced application. The engine disclosed therein includes a unique counterrotating power turbine directly coupled to counterrotating unducted fan blades.
It is important to be able to control the pitch of such propulsor blades. An example of an effective pitch control mechanism is disclosed in U.S. patent application Ser. No. 647,283, Wakeman et al. While the blade pitch varying means disclosed therein is highly reliable, it is desired to further reduce the weight and complexity of such mechanisms.
In one form, the present invention contemplates the use of hydraulic fluid to provide power for changing blade pitch. In order to transmit the fluid between the static engine support structure and the power turbine rotors, it is proposed to use slip rings. Efficient operation of such rings requires relatively small diameters. However, the rotors are supported to the static support by bearings which need a relatively large diameter for stable operation.